In optical transmission systems, in order to absorb loss variation generated in an optical transmission line, ALC that controls output power of an optical amplifier provided in a transmitting station, a repeater station, and a receiving station, to be constant, is adopted. ALC is a control system that monitors the output power of the optical amplifier and performs feed-back control so that a measured value thereof becomes a predetermined target value. By using ALC, even in a case where a loss in the optical transmission line varies due to variations such as a bending loss of an optical fiber or a connector loss, in the optical transmission system after operation has been started once, the optical output can be returned to a constant value, and quality of a signal can be maintained.
Current mainstream optical transmission systems use a WDM system, and the optical amplifier widely used for the system adopts an Erbium Doped Fiber Amplifier (EDFA). The EDFA has a characteristic in that when amplification gain is changed, a wavelength characteristic of the gain changes. Thus, in a setup that performs ALC by adjusting the gain of the EDFA, a uniform gain characteristic cannot be maintained over a whole wavelength range of a WDM signal light. Accordingly, in recent optical amplifiers, Japanese Patent No. 3551418 discloses that a technique has been adopted where the gain of the EDFA is maintained at a constant value by Automatic Gain Control (AGC) so that the wavelength characteristic becomes uniform, and ALC is performed by adjusting the loss by a Variable Optical Attenuator (VOA).
At the time of implementation of ALC, the output power for each wavelength is not controlled to be constant in the WDM signal light in which optical signals of different wavelengths are multiplexed, unless all the wavelengths included in the WDM signal light have a uniform power. Thus, ALC cannot be performed while the number of wavelengths is increased or decreased in the WDM optical transmission system, that is, an addition of the optical signal of an unused wavelength or a deletion of the optical signal of a wavelength being used, and level adjustment for each wavelength needs to be performed, and the implementation of ALC needs to wait until the power of all the wavelengths is adjusted to be uniform. Therefore, Japanese Laid-open Patent Publication No. 2000-196169 discloses that a control system has been proposed where, in a normal state in which the number of wavelengths is determined, ALC by the VOA is performed, while at the time of increasing or decreasing the number of wavelengths the VOA is stopped to perform only AGC of the EDFA, and after the power of all the wavelengths is uniformly adjusted after increment or decrement of the number of wavelengths, ALC by the VOA is resumed.
In optical transmission systems that include an optical amplifier adopting such an ALC as a repeater station, and perform long-distance transmission, when the number of wavelengths is increased or decreased, it is required that there is as little as possible change in the characteristics of the optical power, an optical SN ratio and so on. This point is considered from a point of view of attenuation control of the VOA. At first, at the time of starting increment or decrement of the number of wavelengths, when the optical amplifier in an ALC state is changed over to an AGC state, a target attenuation value of the VOA needs only to be fixed to a value immediately before changeover in the optical amplifier. Accordingly there is no variation in the power of respective wavelengths, causing no major problem.
On the other hand, when ALC is resumed from the AGC state upon completion of increment or decrement of the number of wavelengths, attention is necessary. That is, the target attenuation value of the VOA corresponding to the number of wavelengths after the increment or decrement of the number of wavelengths may be different from the target attenuation value before the increment or decrement of the number of wavelengths. In this case, the attenuation value may be changed, thereby causing a variation in the wavelength power.
In the optical amplifier, the gain of the EDFA is controlled to be constant by AGC, and there is no variation factor other than the attenuation value of the VOA. Thus, if the input power is constant, feed-back control needs only to be performed with respect to the output power. However, if there is a change in the input power, the output power is also changed due to an influence thereof, thereby making the control unstable. Accordingly, at the time of controlling the attenuation value of the VOA, it is necessary to guarantee that the input power of the optical amplifier is stable. In order to guarantee stability of the input power, in a case where the attenuation value of the VOA is changed in optical amplifiers arranged in many repeater stations, that is, in a case where ALC is to be performed, sequence control needs to be performed where, ALC is performed sequentially from an upstream side of optical transmission and proceeds to ALC in the next station after waiting for the output power of an upstream station to converge on a target output value and become stable.
This sequence control is not limited to at the time of increment or decrement of the number of wavelengths, and is also required at the time of startup after a disconnect of signal (recovery). That is, also at the time of startup from the disconnect of signal, the optical output needs to be started up by ALC sequentially from the upstream station. In a downstream station, when an optical signal is output from the upstream station, a disconnect of input in the own station is recovered. However, at this time, unless ALC by the VOA is stable in the optical amplifier in the upstream station, a variation in the output power occurs. Because time is required until the output power is stabilized to the target output value by ALC, if ALC is resumed also in the downstream station immediately after start of optical output in the upstream station, startup is performed while the output power of the upstream station, that is the input power of the downstream station, is still unstable, thereby causing a variation in the wavelength power. Therefore, the sequence control is required similarly to at the time of increment or decrement of the number of wavelengths, where startup of the downstream station waits until ALC in the upstream station becomes stable.
When control speed is taken into consideration for ALC by the VOA in the optical amplifier, it is natural that as the control speed of the VOA increases, time required for stabilizing the power of the entire optical transmission system decreases. However, there is a limitation to speeding up of real-time control by ALC, due to reasons described below.
As described above, AGC is performed with respect to the EDFA in order to make the wavelength characteristic of the gain constant. In the EDFA, because the response time determined by the physical characteristics thereof is comparatively slow of the order of milliseconds (ms), the control speed of AGC is limited by the response time. AGC of the EDFA and ALC by the VOA are operated as a double loop, and when the control speeds of these devices approximate to each other, oscillation may occur. Therefore, the control speed of ALC needs to be 100 times as high as that of AGC, that is, it may not be faster than an order of 0.1 second.
Moreover, a variation in the level at the time of occurring disconnect of signal also limits the speed up of ALC. When a disconnect of signal occurs due to a fault of a light source corresponding to a wavelength channel or a cut of an optical fiber connected to the wavelength multiplexing section or the like, because this is an emergency, increment or decrement of the number of wavelengths occurs during ALC execution in the optical amplifier. For example, in an optical transmission system operated with 10 wavelengths, when 9 wavelengths disappear due to the disconnect of signal, an abrupt variation occurs where the power of the WDM signal light decreases to 1/10. In this case, detection of the disconnect of signal is performed by an electric circuit in a station where the disconnect of signal has occurred, and this is used as a trigger to give a notice of a transition request to AGC to the optical amplifier on a downstream side of the station where the disconnect of signal has occurred. However, because the signal light being transmitted has been transmitted at the velocity of light, the disconnect of signal reaches the downstream station faster than the notice of the transition request to AGC, and during the time lag, execution of ALC is maintained in the optical amplifier in the downstream station. That is, because the optical amplifier in the downstream station does not know that the number of wavelengths has varied due to the disconnect of signal, during the time lag, the optical amplifier determines that the input power has decreased, and performs release control of the VOA to increase the output power up to 10 times. The optical amplifier in the downstream station stops the release control of the VOA later at a point in time when the notice of the transition request to AGC is received. However, when the control speed of ALC is high, release control time during the time lag becomes long because of a quick reaction to the signal light, and hence, a redundant variation in the level occurs in the power of the wavelength, which remains without being cut.
Also in order to suppress an amount of variation in the level at the time of abnormality, ALC may not be performed at high speed. For example, if it is assumed that the time required from detection of the disconnect of signal until a transition request to AGC is transmitted by a transmission section of an Optical Supervisory Channel (OSC) is 30 ms, the time required for photoelectrically converting a notification signal delivered to the downstream station by the OSC to extract the transition request from a payload thereof to AGC, and re-transmitting the notification signal to the next station is 1 ms per one span, and the number of relay spans is 10, then a time of 40 ms is required until the transition request to AGC reaches the receiving station. In order to suppress the variation in the level during this period to 0.05 dB or less, the control speed cs [dB/s] of ALC needs to be: cs[dB/s]×40 ms≦0.05 dB, that is, cs≦1.25 [dB/s].
According to the above-described conditions, the speed of real-time control of the VOA by ALC is set to about 1 dB/s. Actually, it is devised such that when a difference of an actual output value from the target output value is large, the control speed is increased, and when the output value approximates to the target output value, the control speed is decreased. However, an output convergence time required until the attenuation control of the VOA is stabilized, is from 10 to 20 seconds due to the limitation in the control speed of ALC.
Moreover, in the WDM optical transmission system, an optical amplifier having a two-stage configuration in which a Dispersion Compensating Fiber (DCF) is arranged between the EDFAs is frequently used in order to compensate wavelength dispersion in the optical transmission line. In this case, when the VOA is built in to each of the stages, ALC by two VOAs is performed for each station, thereby requiring double time. Moreover, in order to improve the optical SN ratio or enlarge an interval between the repeater stations, a technique is adopted where a Raman optical amplifier using an induced Raman scattering effect in the optical transmission line is also used together for the optical amplifier using the EDFA. In this case, because of a relation where, when Raman pump light is increased, the input power to the EDFA increases, ALC is performed also in the Raman optical amplifier. Therefore, in the case of the optical transmission system including the Raman optical amplifier, the control time associated with ALC for the Raman optical amplifier is added, thereby requiring additional time.
Thus, in the optical transmission system in which the DCF is used for the optical amplifier and the Raman optical amplifier is also used together, when the sequence control for performing ALC sequentially from the upstream side is performed, an output convergence time of from 30 seconds to several minutes is required for one repeater station, and a waiting time for stabilization of this time multiplied by the number of relay spans is required. For example, in a case where the output convergence time by ALC is up to 2 minutes for each station, and there are 10 repeater stations, a waiting time for stabilization of 20 minutes may be required in the entire system.
In an increment or decrement operation of the number of wavelengths or a replacement operation of a faulty wavelength channel, quality of the signal light is ensured by completing ALC after issuing an operation complete instruction. Therefore, a line quality verification test is performed thereafter. At this time, if several tens minutes are required for the waiting time taken for ALC to be stabilized, operation efficiency is poor. Moreover, in a WDM optical transmission system capable of large-capacity transmission, stability of the line and quick recovery from a fault such as power failure are important, and the time of disconnect of signal needs to be as short as possible. That is, in an optical transmission system including many stations, an improvement is desired for the total waiting time for stabilization required for performing ALC sequentially for the number of stations, which has a limitation in the control speed as described above.
In addition to the above-mentioned waiting time taken for ALC to be stabilized, there is also an improvement required for the variation in the level of the signal light. This will be explained with reference to FIG. 4A. In the example illustrated in the figure, three repeater stations provided between a transmitting station and a receiving station are indicated, and it is assumed that each repeater station includes the EDFA and the VOA to perform ALC such as described above. Moreover, a target output value of the wavelength output power of all the stations by ALC is set to 0 dBm/ch.
When each station is in the AGC state at the time of startup or the like, it is assumed that a measured output value of each station is 1 dBm/ch for a first repeater station, −1 dBm/ch for a second repeater station, and 0 dBm/ch for a third repeater station, and ALC is performed in this situation. As described above, because the ALC is started sequentially from the upstream station, first, ALC for the first repeater station is performed. In the first repeater station, because the measured output value during AGC is higher than the target output value by 1 dB, ALC is performed by increasing the target attenuation value of the VOA by 1 dB. In each downstream repeater station at this time, the target attenuation value of the VOA is fixed. Subsequently, after the output convergence time of the first repeater station, ALC is performed for the second repeater station. However, at this time, because the measured output value of the second repeater station becomes −2 dBm/ch due to an influence of ALC of the first repeater station, ALC is performed for the second repeater station by decreasing the target attenuation value of the VOA by 2 dB. Subsequently, after the output convergence time of the second repeater station, ALC is performed for the third repeater station. However at this time, because the measured output value of the third repeater station becomes 1 dBm/ch due to the influence of ALC of the second repeater station, then in the third repeater station the target attenuation value of the VOA is increased by 1 dB.
During sequential execution of ALC with respect to the repeater stations, the power of each wavelength reaching the receiving station varies such that it increases by 2 dB after decreasing by 1 dB, and then decreases by 1 dB. In a receiving station having no margin in the characteristic, there is possibility of causing an error due to the variation in the level. Therefore, it is desired to suppress the variation in the level as much as possible.